Bottom Line:
In particular, we have focused on Silicon On Insulator (SOI) technology, representing the most popular technology for this kind of devices, simultaneously achieving high integration capabilities, small dimensions and low cost.An accurate analysis of single mode behavior for high aspect ratio slot waveguide has been also performed, in order to provide geometric limits for waveguide design purposes.Finally, the problem of coupling into a slot waveguide is addressed and a very compact and efficient slot coupler is proposed, whose geometry has been optimized to give a strip-slot-strip coupling efficiency close to 100%.

ABSTRACTSlot waveguides are becoming more and more attractive optical components, especially for chemical and bio-chemical sensing. In this paper an accurate analysis of slot waveguide fabrication tolerances is carried out, in order to find optimum design criteria for either homogeneous or absorption sensing mechanisms, in cases of low and high aspect ratio slot waveguides. In particular, we have focused on Silicon On Insulator (SOI) technology, representing the most popular technology for this kind of devices, simultaneously achieving high integration capabilities, small dimensions and low cost. An accurate analysis of single mode behavior for high aspect ratio slot waveguide has been also performed, in order to provide geometric limits for waveguide design purposes. Finally, the problem of coupling into a slot waveguide is addressed and a very compact and efficient slot coupler is proposed, whose geometry has been optimized to give a strip-slot-strip coupling efficiency close to 100%.

Mentions:
In order to reduce the effect of the etching residue (t2) on the cladding confinement factor, the influence of the gap dimension has been investigated. In fact, the larger the gap, the smaller the influence of the silicon residue, as evident in Figure 3(b). In particular, the choice of g = 120 nm ensures Ψt2 = −0.2648 %/nm, very close to the value obtained for g = 160 nm (i.e., −0.2493 %/nm), so any practical advantage does not occur by increasing the gap dimension with respect to the etching residue issue. Furthermore, an accurate analysis of the effect of the slanted sidewalls has been performed. The contour map in Figure 3(a) shows the cladding confinement factor versus the gap dimension and the slanted walls angle ϑ. This angle is measured starting from the perfectly vertical sidewalls, as shown in Figure 1. Of course, the effect of the slanted sidewalls is to reduce the surface of the gap region. It could be observed, from Figure 3(a), that the influence of slanted walls is strongly reduced with increasing the gap dimension. For example, for g = 100 nm and ϑ = 8°, Γc decreases with respect to the ideal case (ϑ = 0°) of about 9%, while Γc is reduced by less than 5.4% with g = 160 nm. Accordingly, the absolute value of Ψϑ decreases for wider gaps. In particular, a gap of 100 nm guarantees Ψϑ = −1.24 %/°, while a wider gap g = 160 nm ensures Ψϑ = −0.75 %/°. It should be pointed out that these coefficients represent a worst case estimation. Obviously, the optimal design should depend on the available technology, anyway g = 160 nm has been found as a good trade-off choice since the confinement factor never falls below 42% for ϑ within 10°.

Mentions:
In order to reduce the effect of the etching residue (t2) on the cladding confinement factor, the influence of the gap dimension has been investigated. In fact, the larger the gap, the smaller the influence of the silicon residue, as evident in Figure 3(b). In particular, the choice of g = 120 nm ensures Ψt2 = −0.2648 %/nm, very close to the value obtained for g = 160 nm (i.e., −0.2493 %/nm), so any practical advantage does not occur by increasing the gap dimension with respect to the etching residue issue. Furthermore, an accurate analysis of the effect of the slanted sidewalls has been performed. The contour map in Figure 3(a) shows the cladding confinement factor versus the gap dimension and the slanted walls angle ϑ. This angle is measured starting from the perfectly vertical sidewalls, as shown in Figure 1. Of course, the effect of the slanted sidewalls is to reduce the surface of the gap region. It could be observed, from Figure 3(a), that the influence of slanted walls is strongly reduced with increasing the gap dimension. For example, for g = 100 nm and ϑ = 8°, Γc decreases with respect to the ideal case (ϑ = 0°) of about 9%, while Γc is reduced by less than 5.4% with g = 160 nm. Accordingly, the absolute value of Ψϑ decreases for wider gaps. In particular, a gap of 100 nm guarantees Ψϑ = −1.24 %/°, while a wider gap g = 160 nm ensures Ψϑ = −0.75 %/°. It should be pointed out that these coefficients represent a worst case estimation. Obviously, the optimal design should depend on the available technology, anyway g = 160 nm has been found as a good trade-off choice since the confinement factor never falls below 42% for ϑ within 10°.

Bottom Line:
In particular, we have focused on Silicon On Insulator (SOI) technology, representing the most popular technology for this kind of devices, simultaneously achieving high integration capabilities, small dimensions and low cost.An accurate analysis of single mode behavior for high aspect ratio slot waveguide has been also performed, in order to provide geometric limits for waveguide design purposes.Finally, the problem of coupling into a slot waveguide is addressed and a very compact and efficient slot coupler is proposed, whose geometry has been optimized to give a strip-slot-strip coupling efficiency close to 100%.

ABSTRACTSlot waveguides are becoming more and more attractive optical components, especially for chemical and bio-chemical sensing. In this paper an accurate analysis of slot waveguide fabrication tolerances is carried out, in order to find optimum design criteria for either homogeneous or absorption sensing mechanisms, in cases of low and high aspect ratio slot waveguides. In particular, we have focused on Silicon On Insulator (SOI) technology, representing the most popular technology for this kind of devices, simultaneously achieving high integration capabilities, small dimensions and low cost. An accurate analysis of single mode behavior for high aspect ratio slot waveguide has been also performed, in order to provide geometric limits for waveguide design purposes. Finally, the problem of coupling into a slot waveguide is addressed and a very compact and efficient slot coupler is proposed, whose geometry has been optimized to give a strip-slot-strip coupling efficiency close to 100%.